1. Field of the Invention
The present invention relates to a method for producing a silicon single crystal, and more particularly, to a method for producing a silicon single crystal, the method capable of suppressing a dislocation of a silicon single crystal.
2. Description of the Related Art
There are many methods for producing a silicon single crystal. A typical one among these methods is a Czochralski method (CZ method). FIG. 3 is a typical sectional view showing a single crystal producing apparatus used in the CZ method. In the figure, the reference numeral 1 represents a crucible disposed in a chamber. The crucible 1 is constituted of an internal layer support container 1a made of quartz and having a bottomed cylindrical form and an external support container 1b made of graphite and having the same bottomed cylindrical form, the external support container being fitted so as to support the outside of the internal layer support container 1a. The crucible 1 is secured to the top end of a support shaft 6 which can be rotated and can rise and fall.
A resistance heating type heater 2 is disposed concentrically on the external side of the crucible 1. A melt 13 prepared by melting a fixed amount of raw material by a heater 2 is filled in the crucible 1. A pulling shaft 5 constituted of a wire or a shaft which rotates at a specific speed in a reverse direction or the same direction on the same axis as the support shaft 6 above the center axis of the crucible 1. A seed crystal 15 is allowed to hang from the pulling shaft 5.
When a silicon single crystal is produced using such a single crystal producing apparatus according to the CZ method, first a raw material of silicon is placed in the crucible 1 and the internal pressure in the apparatus is decreased under an inert gas atmosphere. Thereafter, the raw material of silicon is melted by the heater 2 disposed around the crucible 1. After that, the seed crystal 15 hanging from the pulling shaft 5 is immersed in the melt 13 and the pulling shaft 5 is pulled up while rotating the crucible 1 and the pulling shaft 5 to grow a silicon single crystal 12 at the bottom end of the seed crystal 15.
In this CZ method, first a necking portion 12a-forming step of contracting the diameter of the seed crystal 15 up to about 3 mm is performed to remove dislocations involved originally in the seed crystal 15 and dislocations introduced by thermal shock when the seed crystal 15 is dipped in the melt and then a body portion 12c having a fixed diameter is formed through a shoulder portion 12b-forming step of increasing the diameter gradually up to a given crystal diameter.
In the meantime, in the production of a silicon single crystal by the CZ method, a quartz crucible is primarily used as the crucible 1a for storing the silicon melt 13 as aforementioned. When this quartz crucible 1a is brought into contact with the silicon melt 13, its surface is melted to emit oxygen into the melt 13. Oxygen contained in the melt 13 is partly incorporated into the single crystal 12 during pulling-up and has various adverse effects on the quality of a silicon wafer. It is therefore necessary to control the quantity of oxygen to be incorporated into the single crystal 12.
There is, for example, a magnetic-field-applied CZ method as a method of controlling the concentration of oxygen. This method is called a MCZ method (Magnetic-field-applied CZ), in which a magnetic field is applied to the melt, whereby the convection of the melt perpendicular to the line of magnetic force can be limited and controlled. There are various methods as this method of applying a magnetic field and particularly a HMCZ method (Horizontal MCZ) in which a magnetic field is applied in a horizontal direction and a CMCZ method (Cusp MCZ) in which two coils surrounding the furnace body of an apparatus are installed with currents flowing through these coils in the directions opposite to each other to apply a cusp magnetic field are put in practical use.
The foregoing MCZ method has characteristics superior in oxygen density-controllability to those of the CZ method. However, like the CZ method, SiO evaporated from the surface of the silicon melt is cooled and solidified. The solidified SiO falls on the surface of the melt and is incorporated in to the single crystal. Also, the inside wall of the quartz crucible which is in contact with the melt is crystallized and the crystallized portion is peeled off during the growth of the single crystal and incorporated into the single crystal. As aforementioned, these disturbance factors can be avoided incompletely in the MCZ method and a dislocation of the single crystal is frequently caused during the course of the production of the silicon single crystal.
For this reason, when a dislocation of the single crystal is produced in the relatively earlier stage of the production of the silicon single crystal, the single crystal with dislocations is immersed once in the melt and dissolved by controlling the temperature of the melt. Then, the necking step is again started to produce a silicon single crystal newly. It is clear that this production method is desirable in view of productivity.
However, if a silicon single crystal is again produced by dissolving the single crystal with dislocations when this MCZ method is applied to the production of a silicon single crystal having a diameter as large as 200 mm or more by using a large diameter crucible having a diameter of 500 mm or more, this poses the problem that a dislocation of the single crystal occurs frequently in the step of forming the shoulder portion of the single crystal, which significantly decreases the yield of the crystal.
This invention has been made in view of the above situation and it is an object of the present invention to provide a method for producing a silicon single crystal, the method capable of suppressing the dislocation of a single crystal and improving the yield of the silicon single crystal in a MCZ method even if the single crystal with dislocations is dissolved to produce the silicon single crystal again.
A method for producing a silicon single crystal according to the present invention comprises producing a silicon single crystal by a Czochralski method in which a magnetic field is applied, wherein in the case where dislocations are generated in the single crystal during growth, the single crystal with dislocations is dissolved in a nonmagnetic field condition and thereafter a magnetic field is applied again to pull up the silicon single crystal.
In the method for producing a silicon single crystal according to the present invention, the flow rate of argon gas to be supplied to a single crystal producing apparatus is designed to be 100 L/min or more and the pressure in the single crystal producing apparatus is designed to be 6700 pa or less when the dislocated single crystal is dissolved. Further, the number of rotations of the crucible is designed to be 3 rpm or more.
The reason why in the case where the single crystal with dislocations is redissolved while a magnetic field is applied to the melt in the crucible, the dislocation of the single crystal frequently occurs in the subsequent shoulder portion-forming step is considered as follows.
The flow of the melt in the crucible has been clarified by numerical analysis in recent years. For example, in xe2x80x9cCollected Lecture Thesisses of Japan Machinery Institute No. 11 Calculated Dynamics Meeting, (1998), No. 166, pages 419 and 420, the fact is shown that the flow of the melt in the crucible is a roll-like flow running counter to the center plane parallel to the direction of an applied magnetic field in the HMCZ method and an axisymmetric flow rectified in the peripheral direction is present in the CMCZ method. These flows are all directed to the center of the crucible from the wall of the crucible.
Because the temperature of the melt must be raised to dissolve the single crystal with dislocations in general, the evaporation of SiO from the surface of the melt and the dissolution of the inside wall of the quartz crucible which is in contact with the melt are promoted. For this reason, it is considered that in the case of dissolving the single crystal with dislocations in such a condition that a magnetic field is applied to the melt, foreign substances contained in the melt exist as it is in the vicinity of the surface of the melt for a long time, so that they are carried on the flow running towards the center of the crucible from the wall of the crucible and easily reach the growth boundary of the single crystal, causing frequent dislocations.
On the other hand, in xe2x80x9cBARUKU KESSHO SEICHO GIJUTSU (Bulk Crystal Growth Technology) (Baifukan), page 141 to page 143xe2x80x9d, the fact is shown that in a usual CZ method in which no magnetic field is applied, the flow of the melt in the crucible is a flow having a non-axisymmetric eddy structure. Such a flow is considered to render it difficult for the aforementioned foreign substances in the melt to reach the growth surface of the single crystal.
Specifically, if the single crystal with dislocations is dissolved in such a condition that no magnetic field is applied to the melt in the crucible, the condition of the melt which rejects the access of foreign substances on the surface of the melt to the center of the crucible is created. The dislocation of the single crystal can be prevented by pulling the silicon single crystal under the application of a magnetic field in this condition of the melt.
Also, when the single crystal with dislocations is dissolved, it is desirable that the flow rate of argon gas to be supplied to the single crystal producing apparatus be designed to be 100 L/min or more and the pressure in the apparatus be designed to be 6700 pa or less. This accelerates the flow rate of argon gas flowing over the surface of the melt, so that SiO evaporated from the surface of the melt can be efficiently discharged to the outside of the apparatus efficiently and foreign substances floating on the surface of the melt can be driven towards the wall of the crucible.
Moreover, when the single crystal with dislocations is dissolved, the number of rotations of the crucible is desirably 3 rpm or more. By rotating the crucible at a relatively high rotation speed, foreign substances floating on the surface of the melt can be driven towards the wall of the crucible and stuck to the inside wall of the quartz crucible.